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The jfish Peripheral Nerve Stimulator

How does the jfish PNS work?

The jfish PNS circuit is made up of three core parts:

  1. A digital stage comprising a small microcontroller (currently a PICAXE08) running the open-sourced embedded firmware, and serving as the ‘brains’ of the device. The microncontroller controls the pulse patterns, by sensing input from the four user-buttons and responding with output gating of the high voltage stage.
  2. A high voltage analogue stage consisting of a step-up DC-DC converter controlled by a Maxim MAX642 IC. This stage steps up the battery-supplied 9V to the 300V required for maximum current output. The MAX642 uses a small inductor and fast-switching Schottky diodes to store charge from the 9V supply.
  3. A current mirror that ‘mirrors’ the potentiometer-selected current to the output electrodes regardless of the patient’s skin impedance. The output voltage is thus automatically presented by the current mirror, determined by the relationship between the user-selected output current and the skin impedance.

Several other features are implemented as part of these three core components:

  • Battery voltage check provided by the MAX642 IC.
  • LED and piezzo buzzer output to indicate pulse delivery, controlled by the microcontroller outputs.
  • Low-power mode, switching off the high voltage stage, controlled by the microcontroller.

jPNS Schematic

The jPNS schematic is relatively simple. The three core stages outlined in the flowchart can be readily identified. (Clicking on the schematic will open a full-sized version.)

A detailed description of the schematic can be found in the jPNS Design Notes. The PNS change log also contains further description of aspects of the circuit, particularly as these relate to improvements and evolution of the design over time. Both these documents are kept within the project document repository.

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